In the context of gene therapy in a mammal, it is important to monitor the localization of a transgene. Where the transgene encodes a therapeutic polypeptide, such as a protein targeted to kill cancer cells, it is advantageous to have information as to the location, that is, the specific organs, tissues and/or cells which are expressing the polypeptide. There is a need in the art for methods and materials that permit the monitoring of tissue- or cell-specific transgene expression without the requirement to sample and directly test genetically modified cells or tissues.